Arthur Koestler working in his study. Photograph: Pat English/Time & Life Pictures/Getty Image

Arthur Koestler was a man of prodigious appetites. These he applied to everything he did, whether seducing women, writing about politics, or advocating crackpot scientific theories. He once described himself as the "Casanova of causes" and it's true that he embraced a staggering array of beliefs and crusades, ranging from the impressively enlightened (campaigning for euthanasia and against the death penalty) to the downright potty (believing, say, in the benefits of levitation).

Born in Hungary in 1905 to fitfully prosperous Jewish parents, Koestler was educated in Budapest and Vienna. Aged 20, he became a passionate Zionist and a few years later embraced communism, working undercover as an agent throughout the 1930s. He travelled frenetically and lived at one time or another in 13 countries. Captured by Franco's forces while reporting the Spanish civil war in 1937, he narrowly avoided being executed.

When the second world war broke out he was living in France. It was something of a miracle that he avoided Hitler's clutches, and he eventually escaped to Britain in 1940, where he established himself as an intellectual celebrity. He rubbed shoulders with everyone from George Orwell and Cyril Connolly to Jean-Paul Sartre and Timothy Leary, and he had a remarkably active sex life – his several hundred conquests included Simone de Beauvoir.

Koestler didn't just lead an interesting life. He was also a hugely important writer. As with everything else about him, there was a certain inconsistency to his literary efforts. His oeuvre features works of communist propaganda as well as a couple of sex manuals. But at his best he was a masterful, clear-eyed chronicler of the world, someone who combined astonishing learning with a knack for simple, accessible exposition. What are probably his two best-known pieces of writing, his 1940 novel Darkness at Noon and his contribution to Richard Crossman's 1949 essay collection, The God That Failed, were both inspired by his painful renunciation of communism. These works, which remain wonderfully fresh today, were vitally important in helping persuade Europe's left-leaning intelligentsia of the iniquities of the Soviet system. Yet it is in many ways unfair that Koestler's name has become so indelibly associated with them, because it has led to others of his books - in particular his brilliant memoir of his time in a French internment camp, Scum of the Earth - being neglected.

Koestler has long been regarded as an intellectual titan, but it's also true that his reputation has sunk considerably. There are several reasons for this, some to do with changing fashions, others to do with Koestler himself. Today he cuts an oddly remote figure, someone who devoted his life to fighting battles whose importance it is hard for us to fathom. In our apolitical age, his ideological promiscuity looks more like posturing than what it really was, a desperate hankering after the truth.

At the same time, Koestler's reputation has been badly damaged by his own character flaws. There is the controversy surrounding his willingness to let his wife commit suicide with him, even though she was much younger than him and in good health. And there are the allegations that have surfaced concerning his mistreatment of women. Koestler was known to be a sexual bully; De Beauvoir described him as liking rough sex. But in a 1998 biography, David Cesarani added fuel to the fire by alleging that in the 1940s Koestler had raped Jill Craigie, wife of the Labour politician Michael Foot. Although some doubts remain about the allegation, the general consensus is that even if it wasn't true, it wouldn't have been entirely out of character.

Nothing, of course, can ever excuse sexual violence. But there's another, more complex question: to what extent do Koestler's wrongdoings detract from his other, genuinely impressive achievements? While acknowledging his appalling failings as a man, we also, surely, should not entirely blind ourselves completely to the many hugely interesting and important things that he did.